Commercial and military systems often have applications that use optical elements that are subjected to harsh environments. For example, airborne optical imaging systems may use a protective element such as a window or dome that is mounted on an exterior portion of the aircraft to isolate optics and other fragile components of the imaging system from the external environment through which the aircraft is flown. These dome or window elements must be transmissive in the spectral range (such as the infrared spectrum) used in the particular application, have a high degree of resistance to environmental exposures, and have sufficient strength to protect the remaining components of the imaging system during operation.
Conventional materials for windows and domes include single crystal sapphire (alpha aluminum oxide) and aluminum oxynitride (ALON). These materials have a very high degree of strength and a relatively high degree of infrared transparency, particularly over the wavelength range of about 2 micrometers (μm) to about 5 μm. While single crystal sapphire is material of choice in terms of its infrared transparency and hardness, windows made from sapphire are expensive and difficult to fabricate. Sapphire emits radiation at higher temperatures, which is undesirable due to the resulting increase in thermal noise seen by the detector in thermal imaging applications. In addition, sapphire exhibits significantly lower strength as temperature increases, especially above 600° C., which limits its use in hypersonic applications.
More recently, some oxide-based nanocomposite materials, made up of a matrix material containing particles of a nano-dispersoid, have been proposed for optical window and dome applications. For example, a matrix material that can be used for infrared window applications is spinel (MgAl2O4), which has a cubic crystalline structure, high strength, and may be formed into transparent structures. Other oxides can also be used. To strengthen the matrix, as may be required for high impact applications, nano-dispersoid particles that are harder than the matric material can be incorporated to form the nanocomposite material. For mid-wave infrared window applications, the nano-dispersoid particles should be made from a material that is transparent in the infrared spectral range of interest, preferably resistant to oxidation and water, and chemically compatible with the matrix material. Some examples of materials that can be used to make these particles include cubic-boron nitride (c-BN), silicon nitride (Si3N4), silicon carbide (SiC), or a combination of these materials.